As devices manufactured using integrated circuits continue to shrink, the need for smaller packages for the integrated circuit devices continues to increase. One approach increasingly used to save space on a system circuit board and to reduce the board area used is to provide two or more integrated circuits in a combined, vertically arranged package structure called a “Package on Package” or “PoP” device. The PoP structure reduces the system circuit board area needed for the integrated circuits by combining them into a single package structure, and also eliminates the need for some of the connector traces on the circuit board that would otherwise connect the devices to each other. Through via connections may be used to provide electrical connections between the vertically arranged packaged devices.
For example, a memory module may be the device mounted on an upper package in a PoP structure. The memory module could include one, two or more commodity memory devices such as DDR DRAM, or FLASH devices, as non-limiting examples. The upper package substrate may be a multiple level circuit board, and may be formed of a resin, for example woven glass reinforced epoxy resin such as FR4 or BT resin, ceramic, plastic, film, or other substrate materials.
The bottom surface of the upper substrate may have one or more rows of PoP connectors extending vertically away from the bottom surface of the top substrate. These PoP connectors provide the connections to either the integrated circuit mounted on the bottom package of the PoP device, or, to connections that will be mapped to the system board when the PoP device is finally mounted on the system circuit board.
The bottom package is a substrate with an integrated circuit mounted on it. The integrated circuit may be an “application processor” or “AP”. The upper surface of the bottom package has lands or pads for receiving and electrically connecting to the PoP connectors. For example, if the PoP connectors are rows of solder balls extending from the bottom surface of the upper package, lands or pads on the upper surface of the bottom package will correspond to, and receive, those connectors.
The bottom package of the PoP structure will also have external connectors on it, typically on the bottom side, for making the final connection between the PoP structure and the system circuit board. The bottom package may be a ball grid array (“BGA”) type package and have solder balls arranged in an array on the bottom surface. Thus the PoP device has PoP connectors between the top substrate and the bottom substrate, and, external connector terminals extending from the bottom substrate that are mounted on pads on a system circuit board.
However, as the need for higher performance and higher frequency operation devices increases, the routing used in the PoP structures has become a significant limiting factor. The signal paths, which may include board traces, solder balls, solder bumps or C4 connectors, and bond wires, used to get signals from the devices in the PoP structure to and from the system board are quite long. These paths create IR drops and result in slower systems. Further, the use of PoP devices in portable applications increases the need for ever thinner packages.
The drawings, schematics, and diagrams are illustrative and not intended to be limiting, but are examples of embodiments of the invention, are simplified for explanatory purposes, and are not drawn to scale.